We've taught our son that when the light turns red, it is time for bed. When the light is blue, it is time to get up. It has been amazingly beneficial for times of the year like Daylight Saving ending/starting because he's not yet able to read a clock and know "Oh, its 4am - I should try and go back to sleep".

Some children wake early. A light that changes colour lets them know that it's still too early, or that it's now okay to get up.

Commercial lights with timers are available, but they often use blue for night time and I'm not sure if that interferes with sleep or not.

It's a gentle way to sleep train.